This invention relates to methods for controlling the rate of flow of a fluid from a vessel.
There are many instances in which it is necessary to control the rate of flow of fluids from a vessel. Rain water detainment systems, for example, are commonly used to control the rate at which rain water drains from a developed property. The systems generally include a reservoir into which rain water is collected, and one or more drains through which water is discharged from the reservoir. Maximum permissible flow rates out of these systems are generally specified by local regulatory bodies, and are usually keyed to the amount of rainfall.
Rainfall is often expressed in terms of how frequently particular amounts of rainfall would be expected at a particular location. A rainfall that occurs on average once every two years, for example, is referred to as a “two-year storm” or “two-year rainfall”. The probability of such a storm occurring once in a particular year is considered to be 50%, that of such a storm occurring twice in a particular year is considered to be 25%, and so forth. A larger rainfall that occurs on average once every 10 years is referred to as a “10-year storm”—the probability of such a storm occurring in a particular year is 10%. Even larger rainfalls may be categorized as “50-year” or “500-year” rainfalls, for example. Typical regulatory schemes will specify maximum allowable drainage rates in these terms. For example, a code may specify that rainfall equal to or greater than that of a 2-year storm, but less than that of a 10-year storm, may drain from the developed property at the same rate at which rainfall from a 2-year storm would have drained from the undeveloped property. In turn, rainfall equal to or greater than that of a 10-year storm but less than that of a 50-year storm may be drained at the rate at which rainfall from a 10-year storm would have drained from the undeveloped property, and so forth for larger storms. The specifics of the regulatory scheme will vary from jurisdiction to jurisdiction.
The drain(s) controls the rate of water discharge in these detainment systems. By sizing and positioning the drains appropriately, water drainage from the reservoir can be controlled so that it does not exceed a predetermined maximum flow rate. Detainment systems frequently have multiple drains, some of which are not active unless some minimum amount of rainfall is experienced. For instance, a system may have a drain that operates when any rainfall is received, and a second drain that operates only if, for example, rainwater from a 10-year storm is received.
The drains in these detainment systems are typically gravity-fed, open-channel systems. Pumps can be used to manage flow rates from the reservoir, but these increase installation, maintenance and operating costs. In addition, electrical pumps will not operate if power is lost, as often happens during periods of rain because of lightning, winds, automobile accidents and other weather-related causes. Gravity-feed drains are inexpensive, operate passively and can operate effectively for long periods with little maintenance.
The most common type of drain is a simple orifice that allows water to drain from the reservoir to an outlet which is at some lower elevation. Flow rates through the orifice depend on the size of the orifice and the height of the water in the reservoir above the level of the drain. This leads to two seemingly contradictory problems, in which actual flow rates seldom match the desired drainage rate. Very low drainage rates, such as might be desired in the case of small rainfalls being drained from small drainage basins (such as residential lots, small multiple dwelling complexes and small business lots), can only be obtained by making the orifice size very small. Because very small orifices are prone to clogging, many codes specify a minimum orifice size in order to ensure that the system operates efficiently. The result is that when draining small rainfalls from these small drainage basins, the actual drainage rates are higher than desired, because the orifice is too large to restrict the flow to the desired rate.
The converse problem is seen when larger rainfalls are experienced. In this case, drainage rates are maintained at or below the predetermined maximum drainage rate through the size of the orifice. The orifice is sized so that, at the highest water level, the flow rate through the drain is at or below the predetermined maximum. When the water level is lower than the maximum, the flow rate is reduced. This means that for larger storms, the flow rates from the reservoir may be below permitted or desired rates until the reservoir is full. Because the water is not drained as rapidly as permitted, more of it is detained in the reservoir, and the reservoir must be sized to hold that additional water. The result is that the detainment systems must be oversized to hold extra water because the drainage rates are usually less than allowed. Oversizing the system increases equipment, transportation and installation costs.
It would be desirable to provide an inexpensive and reliable system for controlling the rate of discharge of a fluid from a vessel, which permits fluid discharge rates that can be made independent of the fluid level in the vessel, and which permits a wide range of flow rates to be achieved.